Hypodermic syringes are well-known in the art, including pre-sterilized syringes having a protective cap over the cannula or needle, and which is to be removed immediately prior to administering an injection. The cover is intended subsequently to be replaced over the needle after use of the syringe.
However, at this point, the cannula or needle of the syringe has been contaminated by its use on the patient, and, the needle or cannula itself is potentially lethal, in that it then is a carrier of the virus, microvirus, or bacteria. Careless handling, or, in the absence of the protective cap, can result in the user unintentially being pricked, scratched or cut by the sharpened end of the needle, this resulting in infection of the user.
Numerous proposals have been made to avoid this occurrence, but none has been entirely successful.
During use, sudden retraction of the needle into the syringe by spring force, as has been prior proposed, can cause fracturing of the needle within the patient. It can also cause splattering of contaminants on the needle onto the hands of the user, and possibly into the user's eyes in the event that a visor is not worn by the user. Also, the impact of retraction of the needle under spring force can cause an impact on the syringe, resulting in accidental dropping of the syringe, with possible dangerous consequences.
Other prior proposals involve a manually retractable and manually extendable needle guard. However, such needle guards require the use of both hands for them to be forced manually into the extended position in order to break the holding force of a resiliently engaged retaining member. If the user's hand should slip off the needle guard during the extension of the needle guard, there again is the possibility of the user's hand being impaled by the unprotected needle.